Radar determines the distance to an object by directing electromagnetic energy at the object and detecting the presence and character of the energy reflected by the object. The radar principle has been applied from frequencies of a few megahertz to the ultraviolet (laser radar). Since good angular resolution and range resolution are obtainable with radars utilizing coherent radiation, laser radars are useful for target information-gathering applications such as ranging and imaging.
One common use of laser distance measuring devices is in the field of non-contact precision gauging. Devices utilizing a beam of coherent radiation to determine the contour of a target are known. For instance, U.S. Pat. Nos. 3,589,815 and 3,692,414 to Hosterman (issued June 29, 1971 and Sept. 19, 1972, respectively) disclose a non-contacting measuring probe which directs a beam of coherent radiation onto the surface of an arbitrary object. The focal property of a lens is used to determine the distance of the object from the lens. Somewhat similar schemes are disclosed in U.S. Pat. Nos. 3,909,131 to Waters (issued Sept. 30, 1975), 3,986,774 to Lowrey, Jr. et al (issued Oct. 19, 1976), 4,299,291 to Waters et al (issued Nov. 10, 1981) and 4,209,253 to Hughes (issued June 24, 1980). These schemes all have the disadvantage that separate transmitters and receivers must be used, and that the accuracy of the obtained measurements depends upon the mechanical movement of the reflector or a detector.
Interferometry, of course, provides highly accurate measurements of distances from a coherent radiation source to a cooperative reflector. Optical interferometers are well known as tools for metrology and spectroscopy. Measurements of very small distances and thicknesses to a high degree of accuracy may be obtained. See, for example, U.S. Pat. Nos. 4,290,697 to McLandrich (issued Sept. 22, 1981), 4,325,636 to Schiffner (issued Apr. 20, 1982), and 4,329,056 to Lachombat et al (issued May 11, 1982), all of which disclose interferometer devices utilizing an optical path defined by a fiber optic coil. However, interferometry has not been applied to gauging of distances to an arbitrary object (such as a diffuse scattering surface) because the signal-to-noise ratio produced is so low as to pose a major obstacle. See, e.g., U.S. Pat. No. 3,909,131 to Waters, col. 1, lines 29-35.
Nussmeier has developed a self-calibrating interferometer apparatus and method which permits distance measurements to a retro-reflector. This interferometer is disclosed in U.S. Pat. Nos. 4,355,899 and 4,355,900 (both issued Oct. 26, 1982), INTERFEROMETRIC TECHNIQUE FOR MEASURING DISTANCE AT OPTICAL FREQUENCIES (Abstract of Naval Technology from the Air Force Systems Command dated Mar. 28, 1977) and SELF-CALIBRATING INTERFEROMETER FOR OPTICAL PHASE MEASUREMENT (Abstract of New Technology from the Air Force Systems Command, dated Mar. 10, 1977).
Nussmeier's invention permits calculation of an unknown multi-wavelength distance by measuring the differential phase shift between two paths (a known reference path and the unknown path the distance of which is to be measured) at each of several wavelengths. The ambiguity inherent in the measurement of the single wavelength is resolved by combination of the multi-wavelength measurements. A single phase shifter is incorporated within the interferometer. An electromagnetic beam of radiation (preferably a laser beam) is expanded to fill a beam splitter, which divides the beam into a pair of beams (a local beam and a remote beam). The local beam illuminates a reflecting element mounted on the phase shifter. The reflecting element divides the local beam into two portions separated by a slight angle. These two portions of the local beam are reflected back through the beam splitter to a pair of detectors, one for each of the spatially separated portions of the local beam.
The remote beam is also split into two portions by a reference flat which intercepts the remote beam. The reference flat is optically aligned with the remote beam so as to reflect a portion of the remote beam back to the beam splitter (and from there to the first of the detectors). The portion of the local beam reflected from the phase shifter and the portion of the remote beam reflected from the reference flat form interference fringes on the first detector, thereby causing intensity variations as a function of the differential path length.
The remaining portion of the remote beam passes by the reference flat and illuminates the retro-reflector located on the target (which is positioned an unknown distance away from the interferometer). The retro-reflector reflects that portion of the remote beam impinging thereupon back to the beam splitter. The beam splitter directs the reflected remote beam to the second detector, where it interferes with the portion of the local beam impinging upon the second detector and likewise creates interference fringes and associated intensity variations. The transfer function measured from the phase shifter to either detector is sinusoidal with a full cycle for each halfwavelength of phase shifter travel. The phase difference between the two sinusoidal detector outputs represents the optical phase difference between the reference and signal portions (paths) of the remote beam of the interferometer.
In other words, the interferometer in accordance with Nussmeier's invention can be characterized as two Michelson interferometers, one beam length of each of which is controlled by the common phase shifter. The phase shifter disclosed comprises a reflecting element mounted on a conventional translational device (such as piezoelectric element).